Image processing method and device using a line-wise operation

ABSTRACT

Disclosed are an image processing method and device using a line-wise operation. The image processing device, according to one embodiment, comprises: a receiver for receiving an image; at least one first line buffer for outputting the image into a line-wise image line; a first convolution operator for generating a feature map by performing a convolution operation on the basis of the output from the first line buffer; and a feature map processor for storing the output from the first convolution operator in units of at least one line, and processing so as to output the feature map stored in units of at least one line into a two-dimensional form, wherein at least one convolution operation operates in the form of a pipeline.

RELATED APPLICATIONS

This application is a Continuation of U.S. Pat. Application No. 16/960,907 filed on Jul. 9, 2020, which is a National Phase of PCT Patent Application No. PCT/KR2018/015736 having International filing date of Dec. 12, 2018. which claims the benefit of priority of Korean Patent Application Nos. 10-2018-0091485 filed on August 6. 2018 and 10-2018-0005783 filed on Jan. 16, 2018. The contents of the above applications are all incorporated by reference as if fully set forth herein in their entirety.

FIELD AND BACKGROUND OF THE INVENTION

Example embodiments relate to an image processing method and device using a line-wise operation.

Ultra-high-definition (UHD) videos are widely used for UHD television (TV) and Internet protocol TV (IPTV) services and smartphone applications. Although numerous advanced TVs and smartphones support 4 K UHD videos, there are many video streams having a full high-definition (FHD) with a resolution of 1920×1080 due to legacy acquisition devices and services.

Thus, there needs an accurate upscaling method to convert a low-resolution (LR) content to a high-resolution (HR) one. Such an accurate upscaling method may be more needed for video upscaling to convert 2 K FHD to 4 K UHD.

The upscaling method is classified into two types: one is a single image upscaling algorithm that uses a local spatial correlation in a single LR image to reconstruct lost high-frequency details; and the other one is a video upscaling algorithm that uses an additional data dimension or time to improve performance, and costs high for computation.

The single image upscaling algorithm is classified into an interpolation method and a super-resolution (SR) method. The interpolation method uses a simple interpolation kernel such as a bilinear or a bicubic kernel.

The SR method may have improved performance compared to the preceding interpolation method. A fundamental concept of a learning-based approach may be to learn a mapping function from an LR image or video to an HR image or video. The learning-based approach is classified into the following two types.

One is to learn LR-HR mapping by using surrounding information of an LR image, which is based on internal information of an input image. The other one is to perform learning by using an external LR-HR image pair, which is based on external learning or training image, or on a dictionary.

For the SR method, machine learning algorithms such as sparse coding, an anchored neighbor, and a linear mapping kernel have been suggested.

However, such learning-based SR algorithm may need a frame buffer of a great capacity to store intermediate images. Thus, it may not be easy to implement the SR method to convert an HR image to an SR image, for example, an 2 K FHD to a 4 K UHD, in real time in low-complexity hardware (HW).

A recent suggestion, for example, a deep neural network (DNN), and particularly a convolution neural network (CNN), may exhibit great performance in various computer vision fields such as image classification, object detection, and object segmentation.

The machine learning-based method may discover features in a hand-crafted design and learn mappings using such hand-crafted features. In contrast, the DNN may learn itself optimal features and mappings and make overall learning simpler and more effective.

An accurate CNN-based SR method has been suggested to improve a visual quality of HR reconstruction. Such a CNN architecture may include a plurality of layers and nonlinear functions, and be designed to perform SR and generate HR images or high-quality videos.

It is believed that an existing CNN is difficult to be implemented in a low-complexity HW for a real-time application due to excessive multiplications and calculations. In addition, an analysis of a computational complexity and a runtime of the accurate CNN-based SR method is performed at a level of software (SW) of a platform of a central processing unit (CPU) and/or graphics processing unit (GPU). Further, such a CNN architecture may need to use a plurality of frame buffers to store intermediate feature maps when embodied by SW and HW, and thus it may not be easy to be implemented in real time.

SUMMARY OF THE INVENTION

Example embodiments provide a technology for processing an image using a line-wise operation.

According to an example embodiment, there is provided an image processing device including a receiver configured to receive an image, at least one first line buffer configured to output the image as a line-wise image line, a first convolution operator configured to generate a feature map by performing a convolution operation based on an output of the first line buffer, and a feature map processor configured to store an output of the first convolution operator in at least one line-wise unit and process a feature map stored in at least one line-wise unit to be output in a two-dimensional (2D) form. Herein, at least one convolution operation may be performed in a form of a pipeline.

The first convolution operator may be implemented in a residual block that learns a residual signal and outputs the learned residual signal.

The first convolution operator may include at least one one-dimensional (1D) convolution operator configured to perform an 1D convolution operation.

The first convolution operator may include a depth-wise convolution operator and a point-wise convolution operator connected directly to the depth-wise convolution operator.

The feature map processor may include a compressor configured to compress the output of the first convolution operator into at least one line-wise unit.

The feature map processor may further include at least one second line buffer configured to store the output of the first convolution operator that is compressed line-wise.

The feature map processor may further include a decompressor configured to reconstruct an output of the second line buffer to a 2D feature map.

The image processing device may further include a second convolution operator configured to perform a convolution operation based on an output of the feature map processor.

The second convolution operator may include at least one 2D convolution operator configured to perform a 2D convolution operation.

The second convolution operator may include a depth-wise convolution operator and a point-wise convolution operator connected directly to the depth-wise convolution operator.

The image processing device may further include a quantizer configured to quantize a result of at least one convolution operation.

The image processing device may further include a weight buffer configured to store a parameter used for a convolution operation.

According to another example embodiment, there is provided an image processing method including receiving an image, outputting the image as a line-wise image line through at least one first line buffer, generating a feature map by performing a first convolution operation based on an output of the first line buffer through a first convolution operator, and storing an output of the first convolution operator in at least one line-wise unit and processing a feature map stored in at least one line-wise unit to be output in a 2D form through a feature map processor. Herein, at least one convolution operation may be performed in a form of a pipeline.

The first convolution operation may be performed in a residual block that learns a residual signal and outputs the learned residual signal.

The generating may include performing at least one 1D convolution operation.

The generating may include performing a depth-wise convolution operation and performing a point-wise convolution operation directly on a result of the depth-wise convolution operation.

The processing may include compressing the output of the first convolution operator into at least one line-wise unit.

The processing may further include storing, through at least one second line buffer, the output of the first convolution operator that is compressed line-wise.

The processing may further include reconstructing an output of the second line buffer to a 2D feature map.

The image processing method may further include performing a second convolution operation based on an output of the feature map processor.

The performing may include performing at least one 2D convolution operation.

The performing may include performing a depth-wise convolution operation and performing a point-wise convolution operation directly on a result of the depth-wise convolution operation.

The image processing method may further include quantizing a result of at least one convolution operation.

The image processing method may further include storing a parameter used for a convolution operation.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an image processing device according to an example embodiment.

FIG. 2 is a block diagram illustrating a controller illustrated in FIG. 1 .

FIG. 3 is a block diagram illustrating a first convolution operator illustrated in FIG. 2 .

FIG. 4 a illustrates an example of an existing depth-wise separable convolution (DSC).

FIG. 4 b illustrates an example of an operation of the first convolution operator illustrated in FIG. 2 .

FIG. 4 c illustrates another example of an operation of the first convolution operator illustrated in FIG. 2 .

FIG. 5 is a block diagram illustrating a feature map processor illustrated in FIG. 2 .

FIG. 6 a illustrates an example of an operation of the feature map processor illustrated in FIG. 2 .

FIG. 6 b illustrates an example of a compression algorithm of a compressor illustrated in FIG. 5 .

FIG. 7 is a block diagram illustrating a second convolution operator illustrated in FIG. 2 .

FIG. 8 illustrates an example of a hardware architecture of the image processing device illustrated in FIG. 1 .

FIG. 9 illustrates an example of an architecture of a neural network used by the image processing device illustrated in FIG. 1 .

FIG. 10 illustrates an example of a framework for verifying the image processing device illustrated in FIG. 1 .

FIG. 11 a illustrates an example of performance of the image processing device illustrated in FIG. 1 .

FIG. 11 b illustrates another example of performance of the image processing device illustrated in FIG. 1 .

FIG. 11 c illustrates still another example of performance of the image processing device illustrated in FIG. 1 .

FIG. 12 a is an example of an original high-resolution image.

FIG. 12 b is an example of an image processed through a bicubic method.

FIG. 12 c is an example of an image processed through a super-resolution convolutional neural network (SRCNN) method.

FIG. 12 d is an example of an image processed through an SRCNN-Ex method which is an extension of the SRCNN method.

FIG. 12 e is an example of an image processed through a fast SRCNN (FSRCNN) method.

FIG. 12 f is an example of an image processed through an FSRCNN-s method which is a small model size version of the FSRCNN method.

FIG. 12 g is an example of an image processed through a very deep super resolution (VDSR) method.

FIG. 12 h is an example of an image processed by the image processing device illustrated in FIG. 1 using a quantized weight.

FIG. 12 i is an example of an image processed by the image processing device illustrated in FIG. 1 using a quantized weight and activation.

FIG. 12 j is an example of an image processed by the image processing device illustrated in FIG. 1 using a quantized weight and activation, and compression of an intermediate feature map.

FIG. 13 a is another example of an original high-resolution image.

FIG. 13 b is another example of an image processed through a bicubic method.

FIG. 13 c is another example of an image processed through an SRCNN method.

FIG. 13 d is another example of an image processed through an SRCNN-Ex method.

FIG. 13 e is another example of an image processed through an FSRCNN method.

FIG. 13 f is another example of an image processed through an FSRCNN-s method.

FIG. 13 g is another example of an image processed through a VDSR method.

FIG. 13 h is another example of an image processed by the image processing device illustrated in FIG. 1 using a quantized weight.

FIG. 13 i is another example of an image processed by the image processing device illustrated in FIG. 1 using a quantized weight and activation.

FIG. 13 j is another example of an image processed by the image processing device illustrated in FIG. 1 using a quantized weight and activation, and compression of an intermediate feature map.

FIG. 14 illustrates an example of an implementation of super-resolution (SR) hardware by a field programmable gate array (FPGA).

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

Hereinafter, example embodiments will be described in detail with reference to the accompanying drawings. However, various changes, modifications, and equivalents of the methods, apparatuses, and/or systems described herein will be apparent after an understanding of the present disclosure of this application. For example, the sequences of operations described herein are merely examples, and are not limited to those set forth herein, but may be changed as will be apparent after an understanding of the present disclosure, with the exception of operations necessarily occurring in a certain order.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, integers, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, operations, elements, components, and/or groups thereof.

Terms such as first, second, A, B. (a), (b), and the like may be used herein to describe components. Each of these terminologies is not used to define an essence, order, or sequence of a corresponding component but used merely to distinguish the corresponding component from other component(s). For example, a first component may be referred to as a second component, and similarly the second component may also be referred to as the first component.

Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains based on an understanding of the present disclosure. Terms, such as those defined in commonly used dictionaries, are to be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and are not to be interpreted in an idealized or overly formal sense unless expressly so defined herein.

In addition, regarding the reference numerals assigned to the elements in the drawings, it should be noted that the same elements will be designated by the same reference numerals, wherever possible, even though they are shown in different drawings. Also, in the description of embodiments, detailed description of well-known related structures or functions will be omitted when it is deemed that such description will cause ambiguous interpretation of the present disclosure.

The term “module” described herein may indicate hardware to perform a function and an operation of each component described herein, a computer program code to perform a certain function and operation, or an electronic recording medium, for example, a processor and a microprocessor, in which a computer program code used to perform a certain function and operation is provided.

That is, a module described herein may indicate hardware to perform technical ideas of the present disclosure, and/or a functional and/or structural combination with software to operate the hardware.

FIG. 1 is a block diagram illustrating an image processing device according to an example embodiment.

Referring to FIG. 1 , an image processing device 10 may receive an image and process the received image. For example, the image processing device 10 may process the received image to generate a high-resolution (HR) image.

The image processing device 10 may effectively process the received image in low-specification hardware. The image processing device 10 may train a neural network based on the received image.

The image processing device 10 may increase a resolution of the image using the trained neural network. For example, the image processing device 10 may perform super resolution (SR) on a low-resolution (LR) image.

The image processing device 10 may provide a hardware-friendly convolutional neural network (CNN)-based SR method. The image processing device 10 may be embodied in a field programmable gate array (FPGA) to convert a 2 K full high-definition (FHD) image to a 4 K ultra-high-definition (UHD) one at 60 frames per second (fps).

The image processing device 10 may effectively perform the SR using a neural network in hardware having a limited computation and memory space. The image processing device 10 may process an LR input line-wise, and maintain a parameter value of a convolution filter at a small number. That is, the image processing device 10 may process LR data line-wise, and thus significantly reduce the number of filter parameters compared to an existing CNN.

The image processing device 10 may process an SR image using a cascade of a one-dimensional (1D) convolution. The image processing device 10 may thus save a required line memory by maintaining a large receptive field along a horizontal line while maintaining a vertical receptive field at a small size. Herein, a line memory may include a line buffer.

The image processing device 10 may reduce the number of filter parameters of the neural network by combining a residual connection and a depth-wise separable convolution (DSC), and maintain improved SR performance with a reduced quantity of computations or operations.

The image processing device 10 may convert 32-bit floating-point data to fixed-point data without a degradation of a peak signal-to-noise ratio (PSNR), through quantization. In addition, the image processing device 10 may compress a feature map to reduce a line memory needed to store feature map data.

The image processing device 10 includes a receiver 100 and a controller 200.

The receiver 100 may receive an image. The image received by the receiver 100 may indicate an image of an object formed by refraction or reflection of light. The image may include, for example, a video, a picture, a photograph, and the like.

The receiver 100 may receive the image in a form of pixel information. For example, the receiver 100 may receive two-dimensional (2D) pixel information.

The image may include an LR image and an HR image.

The controller 200 may process the received image. The controller 200 may increase a resolution of the received image. The controller 200 may perform the SR on the received image.

The controller 200 may process the image line-wise. The controller 200 may output the image line-wise and perform a line-wise operation to increase a resolution of the image.

FIG. 2 is a block diagram illustrating the controller illustrated in FIG. 1 .

Referring to FIG. 2 , the controller 200 includes a first line buffer 210, a first convolution operator 220, and a feature map processor 230. The controller 200 also includes a second convolution operator 240. a quantizer 250, and a weight buffer 260.

The first line buffer 210 may output an image as a line-wise image line. The first line buffer 210 may include one or more line buffers. Here, the number of the line buffers may be determined based on a size of a convolution filter.

The first convolution operator 220 may generate a feature map by performing a convolution operation based on an output of the first line buffer 210.

The feature map processor 230 may store an output of the first convolution operator 220 in at least one line-wise unit, and process a feature map stored in at least one line-wise unit to be output in a 2D form.

The second convolution operator 240 may perform a convolution operation based on an output of the feature map processor 230. That is, the second convolution operator 240 may perform the convolution operation based on a feature map output in the 2D form.

The quantizer 250 may quantize a result of at least one convolution operation. The quantizer 250 may quantize convolution results and filter parameters using various quantization methods.

The quantization methods of the quantizer 250 may include all quantization algorithms that convert a floating point to a fixed point, for example, a uniform quantization and a nonuniform quantization. For example, the quantizer 250 may quantize the convolution results and the filter parameters through the uniform quantization.

The image processing device 10 may use a fixed-point representation through the quantizer 250 to reduce complexity. The quantizer 250 may convert floating-point data to fixed-point data.

The fixed-point data may be defined as [IL. FL] to represent respective numerical values, in which IL denotes an integer length and FL denotes a fraction length. The quantizer 250 may calculate a total bit number used to represent a numeral by adding an integer bit number and a fraction bit number.

A sum of these, or IL + FL, may be represented by a WL which denotes a word length. The quantizer 250 may limit a level of data precision to an FL, bit through a fixed-point form in [IL, FL], and set a range to be [-2^(IL-1), 2^(IL-1)-2^(-FL)].

When converting a floating point to a fixed point, the quantizer 250 may use a round-off method that rounds off to the nearest. Such a round-off method may be represented by Equation 1.

$\begin{matrix} {Round\left( {x,\left\lbrack \text{IL, FL} \right\rbrack} \right) = \left\{ \begin{array}{ll} \left\lfloor x \right\rfloor & {\text{­­­[Equation 1]}\left\lfloor x \right\rfloor \leq x \leq \left\lfloor x \right\rfloor + \frac{\varepsilon}{2}} \\ {\left\lfloor x \right\rfloor + \varepsilon} & {\text{if}\left\lfloor x \right\rfloor + \frac{\varepsilon}{2} \leq x \leq \left\lfloor x \right\rfloor + \varepsilon} \end{array} \right)} &  \end{matrix}$

In Equation 1. x is defined as a greatest integer multiple of ε(=2⁻ ^(FL)), which is less than or equal to x. If x is out of a range of [IL, FL], a result may be saturated to a lower or upper limit of [IL, FL]. A method of converting a floating point to a fixed point may also be represented by Equation 2.

$\begin{matrix} {Convert\left( {x,\left\lbrack \text{IL,FL} \right\rbrack} \right) = \left\{ \begin{array}{l} {\text{- 2}^{\text{IL-1}}\quad,\mspace{6mu}\text{if}x \leq \text{-2}^{\text{IL-1}}} \\ {2^{\text{IL-1}} - 2^{- \text{FL}}\quad,\text{if}x \geq 2^{\text{IL-1}}\text{- 2}^{\text{-FL}}} \\ {Round\left( {x,\left\lbrack \text{IL,FL} \right\rbrack} \right),\text{otherwise}\text{.}} \end{array} \right)} & \text{­­­[Equation 2]} \end{matrix}$

To minimize a PSNR degradation of a test set of floating-point data (filter parameters and activation values) through numerous experiments, optimal WL, IL, and FL values may be applied to the image processing device 10. Here, a degradation by the quantization in a network used by the image processing device 10 may be extremely insignificant.

The weight buffer 260 may store a parameter used for a convolution operation.

The components described above may operate through a pipeline. For example, at least one convolution operation may operate through a pipeline.

FIG. 3 is a block diagram illustrating the first convolution operator illustrated in FIG. 2 .

Referring to FIG. 3 , the first convolution operator 220 may be implemented in a residual block that learns a residual signal and outputs the learned residual signal. The first convolution operator 220 may include at least one 1D convolution operator configured to perform a 1D convolution operation.

The 1D convolution operation may indicate a convolution operation using line-wise data as an input. For example, the 1D convolution operation may indicate a 1×n convolution. In this example, n, which denotes a length of the line-wise data, may have an arbitrary integer value greater than or equal to 2.

The first convolution operator 220 includes a depth-wise (DW) convolution operator 221 and a point-wise (PW) convolution operator 223 connected directly to the DW convolution operator 221.

The DW convolution operator 221 may perform a convolution operation in a depth direction of a feature map. The DW convolution operator 221 may perform a DW convolution operation one or more times. The PW convolution operator 223 may perform a convolution operation for each point. The PW convolution operator 223 may perform a PW convolution operation one or more times.

The DW convolution operator 221 may include at least one 1D convolution operator. The PW convolution operator 223 may perform a 1 × 1 convolution.

FIG. 4 a illustrates an example of an existing DSC. FIG. 4 b illustrates an example of an operation of the first convolution operator illustrated in FIG. 2 . FIG. 4 c illustrates another example of an operation of the first convolution operator illustrated in FIG. 2 .

Referring to FIGS. 4 a through 4 c , the first convolution operator 220 may use a DSC to perform a convolution operation. Through this, the first convolution operator 220 may achieve a similar classification performance only with the number of parameters that is approximately ⅑ of that of an existing non-separable convolution.

The DSC may include a DW convolution operation, a rectified linear unit (ReLU), and a PW convolution operation that are connected in cascade.

However, the DSC may have a relatively low performance when being used in regression as in SR. A batch normalization (BN) may require a relatively high operation or computation quantity to calculate a mean and a variance, while degrading a performance in a regression analysis.

Thus, the first convolution operator 220 may use an architecture in which the BN and the ReLU are removed from the DSC. For example, the first convolution operator 220 may remove the ReLU between the DW convolution operator 221 and the PW convolution operator 223, and use them through a direct connection therebetween.

When a lower number of convolution filters is used along with the ReLU in the DSC, feature maps passing through the ReLU may become extremely sparse. This may obstruct learning and result in a lower PSNR performance.

FIG. 4 a illustrates an existing DSC architecture, and FIG. 4 b illustrates an architecture of a convolution layer used by the first convolution operator 220. A result of comparing PSNR and structural similarity (SSIM) performances for a dataset. Set-5, when there is the ReLU between the DW convolution operator 221 and the PW convolution operator 223 and when there is not the ReLU between the DW convolution operator 221 and the PW convolution operator 223. is as illustrated in Table 1 below.

TABLE 1 Mean PSNR Mean SSIM ReLU present 33.54 dB 0.9544 ReLU absent 33.66 dB 0.9548 Difference + 0.12 dB + 0.0004

In the existing DSC, a 3×3 size filter may be used for a DW convolution. However, some display applications such as T-Con may not excessively use a line memory, and thus there may be a restriction on using such a 3×3 size filter in a network. However, to obtain a high performance in deep learning, a large receptive filter using a 3×3 or greater filter may be needed.

Thus, the image processing device 10 may use a 1D horizontal convolution for some convolution layers such that a network is more compact and suitable for hardware to which LR input data is to be streamed line-wise. For example, the image processing device 10 may apply a 1D horizontal convolution to the first convolution operator 220.

Thus, the first convolution operator 220 may have a rectangular receptive field having a longer length in a horizontal direction and a shorter length in a vertical direction. Thus, the image processing device 10 may reduce a line memory required to store intermediate feature maps to be as small as possible.

For effective hardware implementation, a convolution filter with such a small network may need to be maintained. However, when filter parameters are less, learning or training of a network including the DSC and the 1D horizontal convolution may be degraded.

This is because an interlayer connection in the network may be sparse due to the shortage of filter parameters, and thus learning of image reconstruction or restoration may not be performed properly.

The image processing device 10 may considerably reduce the number of filters while exhibiting a desirable SR performance by inserting a residual connection into a network. For example, the image processing device 10 may reduce the number of filters by implementing the first convolution operator 220 in a residual block.

In terms of hardware, additional line memories may be needed to store an input of the residual connection in order to embody the residual connection by a 2D convolution, and be needed at ends of the connection.

Thus, the image processing device 10 may be readily implemented in hardware by using a delay buffer using only a 1D convolution in the residual connection. FIG. 4 c illustrates a final DSC architecture having a final 1D horizontal convolution and a residual connection.

FIG. 5 is a block diagram illustrating the feature map processor illustrated in FIG. 2 .

Referring to FIG. 5 , the feature map processor 230 includes a compressor 231, a second line buffer 233, and a decompressor 235.

The compressor 231 may compress an output of the first convolution operator 220 into at least one line-wise unit. That is, the compressor 231 may compress a feature map into a least one line-wise unit. The compressor 231 may compress the feature map in at least one direction of a width direction, a height direction, or a depth direction of the feature map.

The second line buffer 233 may store the output of the first convolution operator 220 that is compressed line-wise. That is, the second line buffer 233 may store the feature map that is compressed line-wise. The second line buffer 233 may include one or more line buffers. The number of the line buffers may be determined based on a size of a convolution filter to be used to perform a convolution operation.

The decompressor 235 may reconstruct an output of the second line buffer 233 to a 2D feature map. That is, the decompressor 235 may reconstruct the feature map that is compressed line-wise to the 2D feature map.

FIG. 6 a illustrates an example of an operation of the feature map processor illustrated in FIG. 2 . FIG. 6 b illustrates an example of a compression algorithm of the compressor illustrated in FIG. 5 .

Referring to FIGS. 6 a and 6 b , the compressor 231 may compress a feature map through various compression algorithms. A compression algorithm may include fixed-length coding and variable-length coding. The fixed-length coding may include a S3 Texture Compression (S3TC) (sometimes also called DXT) algorithm, and a block-based algorithm such as joint photographic experts group (JPEG) and JPEG 2000. The fixed-length coding may be preferred in terms of hardware complexity.

The variable-length coding may include Huffman coding and arithmetic coding. The variable-length coding may be used to increase a compressibility.

In addition, the compressor 231 may compress the feature map using an algorithm modified from the algorithms described in the foregoing.

A size of a receptive field may greatly affect performance. In addition, both a horizontal receptive field and a vertical receptive field may be important. However, in a case of a 3×3 convolution including a vertical direction, feature map data may need to be stored in a line memory when data output after passing through a previous convolution layer is transmitted to a subsequent 3×3 convolution layer.

When passing through a 3×3 convolution layer, line memories that are twice as many as the number of line memories required to store output feature maps of a current layer may be needed.

However, using many line memories may result in issues in chip design. For example, the issues may include an increase in chip size due to an increase in the number of power rings used in the line memories, a routing congestion in place and route (P&R), and a voltage drop in case of a shortage of power rings in a memory block boundary.

To solve such issues described in the foregoing, there may need a method of reducing the number of line memories, and a feature map compression method may be considered.

The compressor 231 may use the feature map compression method based on various considerations in terms of hardware implementation. A feature map to be compressed by the compressor 231 may include an intermediate feature map.

The compressor 231 may use an extremely simple compression algorithm. Herein, compression may be used to reduce the number of line memories. Thus, a size of a logic used for the compression may need to be smaller than a size of a memory required to store an intermediate feature map before the compression.

Residual learning and use of a ReLU may cause many 0 s and values near to 0 in feature maps, and thus the compressor 231 may provide an effective compression algorithm based on such a data characteristic. The compressor 231 may compress data, using only nearby data in a horizontal direction to effectively use a line memory.

For example, an algorithm used by the compressor 231 may include an algorithm modified from a DXT5 algorithm to be suitable for a CNN architecture. For example, when compressing RGB pixels of a 4×4 block, DXT5 may compress respective RGB color channel inputs, independently.

In this example, a maximum value (MAX) and a minimum value (MIN) of each color channel may be calculated. Thus, six intermediate points may be generated through interpolation using the calculated MAX and the MIN. The MAX and MIN values, and the six intermediate points may be defined as a reference value for the compression.

To encode pixel data, an index value of a color nearest to a reference color may be assigned to each pixel. The encoding may be completed by storing a 4×4 block index value, and the MAX and MIN values. There may be eight neighboring index values for each pixel in a 4×4 block, and each index may be represented by 3 bits for each pixel.

In addition, decoding may be readily performed using the MAX and MIN values, and the index value, in reverse order from that of the encoding. For example, when a bit per pixel (bpp) of an RGB input is 8 bits, DXT5 may have a set compression ratio (CR) of 2:1 for the 4×4 block.

The CR may be calculated as represented by Equation 3.

$\begin{matrix} \begin{matrix} {CR = \frac{\text{uncompressed bits}}{\text{compressed bits}}} \\ {= \frac{\left( \text{b} \right|\text{pp} \times \text{block\_size}}{\text{3} \times \left( {\text{max + min + block\_size} \times \text{index}} \right)}} \end{matrix} & \text{­­­[Equation 3]} \end{matrix}$

The compressor 231 of the image processing device 10 may provide a compression method that minimizes an image quality degradation and increases a CR by modifying the DXT5 algorithm. Differences between the existing DXT5 and the image processing device 10 are as indicated in Table 2 below.

TABLE 2 Method Existing DXT5 Image processing device 10 Input RGB Intermediate feature maps Bits 24 bits 14 bits (quantized) Block 4×4 1×32 Max value Compute Compute Min value Compute 0 (fixed) Bits per index 3 5 Divisor value 7 32 (approximate) Compression ratio 2:1 2.58:1

The compressor 231 may calculate only the maximum value with the minimum value being set as 0. By setting the minimum value as 0, the feature map processor 230 may use a characteristic of data of intermediate feature maps being 0 or a value near to 0.

By setting the minimum value as 0. the image processing device 10 may reduce bits to store the minimum value, and remove a logic to calculate the minimum value. Since the data of the intermediate feature maps needs to be processed line-wise in hardware, a block size of the data of the feature maps may be set to be 1×32.

In addition, a 5-bit index may be assigned, as a quantization level, to each set of data in the 1×32 block of the feature maps. A single index of the data may be assigned to maintain an image quality. A 5-bit length for indices may be experimentally determined by verifying a PSNR performance based on a bit length for data point indices.

A CR of the compressor 231 may be represented by Equation 4.

$\begin{matrix} {PCR = \frac{\text{bits of quantized feature map} \times \text{block\_size}}{\left( {\text{bits of max + block\_size} \times \text{bits of index}} \right)}} & \text{­­­[Equation 4]} \end{matrix}$

For example, when a word length (WL) of feature map data after quantization of an activation is a 14-bit depth, the CR may be 2.58:1(=14(1×32)/(14+5(1×32))). That is, the number of line memories to store feature maps may be reduced to approximately 2.58 times.

As indicated in Table 2, the compressor 231 may set a divisor value to be 32, which is a multiple of 2, in lieu of 31(2⁵-1), to reduce hardware complexity in calculating intermediate points. Thus, the compressor 231 may calculate intermediate points by shift and add operators.

FIG. 7 is a block diagram illustrating the second convolution operator illustrated in FIG. 2 .

Referring to FIG. 7 , the second convolution operator 240 includes a DW convolution operator 241 and a PW convolution operator 243 connected directly to the DW convolution operator 241. The second convolution operator 240 may include at least one 2D convolution operator configured to perform a 2D convolution operation.

The DW convolution operator 241 may perform a convolution operation in a depth direction of a feature map. The DW convolution operator 241 may perform a DW convolution operation one or more times. The PW convolution operator 243 may perform a convolution operation for each point. The PW convolution operator 243 may perform a PW convolution operation one or more times.

The 2D convolution operation may indicate a convolution operation using 2D data as an input. For example, the 2D convolution operation may indicate an m×n convolution. In this example, m and n, which denote a length of line-wise data, may have an arbitrary integer value greater than or equal to 2.

Although a 3×3 convolution is provided herein as an example of the 2D convolution, the 2D convolution used by the image processing device 10 is not limited to such an example convolution.

The DW convolution operator 241 may perform a convolution operation in a depth direction of a feature map, whereas the PW convolution operator 243 may perform a convolution operation for each point.

The DW convolution operator 241 may include at least one 2D convolution operator. The PW convolution operator 243 may perform a 1×1 convolution.

Similar to the first convolution operator 220, the second convolution operator 240 may also perform a convolution operation using a DSC. For a detailed description of the DSC, reference may be made to the description of the DSC provided above with respect to the first convolution operator 220.

FIG. 8 illustrates an example of a hardware architecture of the image processing device illustrated in FIG. 1 .

The image processing device 10 may process an LR image to generate an HR image. For example, the image processing device 10 may generate a 4 K UHD image from an FHD image.

FIG. 8 illustrates a pipeline hardware architecture for SR, which is designed by two types—Type 1 without compression of intermediate feature maps, and Type 2 with the compression.

Details of the example illustrated in FIG. 8 are as indicated in Table 3 below.

TABLE 3 Type/Stride/Padding Filter Shape Input/Output Size Remarks 1920×1080×1 Input Y Conv/ (1,1) / (1,1) 3×3×1×32 1920×1080×32 ReLU - 1920×1080×32 DW Conv / (1,1) / (0,2) 1×5×32 dw 1920×1080×32 Residual Block PW Conv/ (1,1) / (0, 0) 1×1×32×16 1920×1080×16 ReLU - 1920×1080×16 DW Conv / (1,1) / (0,2) 1×5×16 dw 1920×1080×16 PW Conv / (1,1) / (0,0) 1×1×16×32 1920×1080×32 ReLU - 1920×1080×32 DW Conv / (1,1) / (1,1) 3×3×32 dw 1920×1080×32 PW Conv / (1,1) / (0, 0) 1×1×32×16 1920×1080×16 ReLU - 1920×1080×16 DW Conv/(1,1)/(1, 1) 3×3×16 dw 1920×1080×16 PW Conv / (1,1) / (0,0) 3×3×16×4 1920×10804 Pixel Shuffle depth-to-space 3840×2160×1 Y_(C) Nearest Neighbor 2× up-sample 3840×2160×1 Y_(N) Residual Network Y_(N) + Y_(C) 3840×2160×1 Output Y_(F)

As illustrated, the image processing device 10 includes a first pixel information converter, the first line buffer 210. a data aligner, the DW convolution operator 221, the PW convolution operator 223, the compressor 231, the second line buffer 233. the decompressor 235. the DW convolution operator 241, the PW convolution operator 243. the quantizer 250, the weight buffer 260. a second pixel information converter, and a third line buffer.

An image received by the first pixel information converter may include color data. For example, the color data may include RGB channel data and YCbCr channel data.

The first pixel information converter may convert first color data to second color data. The first color data may include RGB channels, and the second color data may include YCbCr channels. For example, the first pixel information converter may convert RGB channels of an LR input image to YCbCr channels.

The first line buffer 210 may include four line buffers, for example. The DW convolution operator 221 may perform a 1×5 convolution operation. The second line buffer 233 may include an even-number line buffer and an odd-number line buffer.

The DW convolution operator 241 may perform a 3×3 convolution operation. The second pixel information converter may convert the second color data to the first color data. For example, the second pixel information converter may convert the YCbCr channels to the RGB channels.

The weight buffer 260 may store a parameter, or a filter parameter, that is used for a convolution operation. The weight buffer 260 may update parameters received from convolution operators.

The third line buffer may include a plurality of line buffers. The third line buffer may include four output line buffers, for example.

Outputs of all the convolution operators may be quantized through the quantizer 250. and all weight parameters may also be quantized through the quantizer 250.

The quantizer 250 may convert a 32-bit floating point to a 10-bit fixed point. The weight buffer 260 may store the quantized weight parameters.

In FIG. 8 , arrows indicate respective data paths. That is, illustrated are a data path based on Type 1, and a data path based on Type 2.

The image processing device 10 may operate in a pipeline structure. The pipeline structure may indicate a structure in which an output in one step of processing data is connected to an input in a next step of processing the data. Connected steps of processing data may be performed concurrently or in parallel.

That is, the components included in the image processing device 10 may operate concurrently or in parallel to process an image. For example, at least one convolution operation of the image processing device 10 may operate in a form of a pipeline.

The convolution operators may load convolution filter parameters from the weight buffer 260. Subsequently, the first pixel information converter may extract a YCbCr value from an RGB input stream. The first line buffer 210 may store four rows of an YCbCr LR input image to be used for nearest neighboring point upscaling to obtain an interpolated image for a residual connection at an end of a network.

The data aligner may re-align data of the four line buffers of the first line buffer 210 and the input stream, and generate 3×3 size YCbCr LR patches. A Y channel of the LR patches may be transmitted to a 3×3 convolution layer.

After a first convolution operation, a feature map may pass through a ReLU activation function. Subsequently, an output of the ReLU function may pass through the first convolution operator 220. The first convolution operator 220 may generate a feature map, or an intermediate feature map.

The compressor 231 may compress the intermediate feature map that has passed through a residual block and a ReLU, and the second line buffer 233 may store the compressed feature map.

The decompressor 235 may read data stored in the second line buffer 233. and decompress the read data at a one-delayed line data-enable (DE) timing. The DW convolution operator 241 may perform a 3×3 convolution operation on the decompressed data, and the PW convolution operator 243 may perform a 1×1 convolution operation on an output of the DW convolution operator 241.

After an output of the PW convolution operator 243. the number of channels of a feature map may be reduced by half from 32 to 16. The feature map with the reduced number of channels may pass again through the compressor 231, the second line buffer 233, and the decompressor 235 in sequential order. Subsequently, convolution operations may be performed again by the DW convolution operator 241 and the PW convolution operator 243.

An output of such repeated convolution operations may be configured by four channels to be used to generate a 2×2 HR patch through a similar method as that used for a sub pixel convolution.

The image processing device 10 may then obtain a final Y (Y_(F)) by adding 2×2 super-resolved Y data (Yc) and 2X up-sampled data (Y_(N)) through a nearest neighbor interpolation method.

To synchronize two timings of Y_(C) and Y_(N), the Y_(N) data may be stored first-in, first-out (FIFO), and read at a same timing as Yc. In addition, CbCr data delayed from the FIFO may also be up-sampled by two times based on the nearest neighbor interpolation method to be transmitted to the second pixel information converter to obtain RGB pixels.

Two output buffers of the third line buffer may store generated 2×2 RGB HR patches, which may be transmitted to a display device at an output timing for each output clock cycle.

To prevent a read/write collision for the 2×2 RGB HR patches using a dual buffering structure for stream processing, four line buffers may be used as the third line buffer.

FIG. 9 illustrates an example of an architecture of a neural network used by the image processing device illustrated in FIG. 1 .

Referring to FIG. 9 , the image processing device 10 may process an image using a hardware-friendly CNN-based SR network.

The image processing device 10 may process the image using only a portion of color data. For example, the image processing device 10 may process the image by inputting, to the CNN network, only a luminance signal channel (Y) among YCbCr channels. A level of performance of learning using only the Y channel may be similar to a level of performance of learning using RGB channels.

When training the CNN network with the RGB channels, the number of parameters used for the RGB channels may be three times greater than the number of parameters used only for the Y channel in a 2D convolution of a first layer and a PW convolution of a last layer.

Here, color difference signal (Cb. Cr) channel data may be up-scaled using an interpolation method. The interpolation method may include a bicubic interpolation and a nearest neighbor interpolation.

For example, the image processing device 10 may perform the up-scaling using the nearest neighbor interpolation which may be simpler than the bicubic interpolation to reduce complexity and improve hardware efficiency. In addition, the image processing device 10 may train the neural network using a residual learning technique to reduce the complexity.

The image processing device 10 may calculate a final HR image Y_(F) by adding an interpolated LR image Y_(N) and an output Yc of the network. The calculation may be represented by Equation 5.

$\begin{matrix} {\text{Y}_{F} = \text{Y}_{N} + \text{Y}_{C}} & \text{­­­[Equation 5]} \end{matrix}$

To use as less convolution filter parameters and line memories as possible, the image processing device 10 may combine a DSC, an 1D horizontal convolution, and a residual connection.

Thus, the number of filter parameters may be approximately 21 times less than an existing extension of a super-resolution convolutional neural network (SRCNN) (SRCNN-Ex), approximately 4.5 times less than a fast SRCNN (FSRCNN), and approximately 1.56 times less than a small model size version of an FSRCNN (FSRCNN-s), while levels of PSNR and SSIM performance may be similar to those of the SRCNN-Ex.

As described above with reference to FIG. 9 , the image processing device 10 may perform a convolution operation through two 2D convolution layers and one 1D convolution layer. For example, a 2D convolution operation may be a 3×3 convolution operation, and the 1D convolution operation may be a 1×5 convolution operation. In this example, a total receptive field size may be 7×15.

FIG. 10 illustrates an example of a framework for verifying the image processing device illustrated in FIG. 1 .

Referring to FIG. 10 , to compare the image processing device 10 to a bicubic method and an existing CNN-based SR method, performance is evaluated with respect to a general dataset. Through such a performance evaluation, performance of the image processing device 10 may be compared to those of software-based methods including, for example, SRCNN, SRCNN-Ex, FSRCNN, FSRCNN-s, and very deep SR (VDSR). In addition, performance of the image processing device 10 may be compared to those of other real-time SR hardware in terms of gate count and operating frequency.

A generally used benchmark dataset may be used for learning and tests. An SR network may be trained or learned using 291 images including 91 images from Yang et al. and 200 images from Berkeley segmentation dataset.

For the comparison of performances, test set 1 and test set 2 may be used. Test set 1 may include Set5, Set 14, B 100, and Urban 100, which may be frequently used as an SR benchmark for many methods. Test set 2 may include eight 4 K UHD images, and used for tests.

All tests or experiments may be performed with a scale factor of 2 times for SR. A PSNR and an SSIM may be used as a measure or a metric for the evaluation. The SR may be performed for a luminance channel of a YCbCr color space, and thus the PSNR and the SSIM may be calculated using a Y channel of a reconstructed original HR image.

For learning and tests, an LR input image may be intentionally generated through down-sampling from the original HR image using a double scale bicubic interpolation. For learning, 128×128 size sub-images may be randomly cropped. Through rotation, reflection, and scaling, an LR-HR learning image pair may be augmented.

Weights may be initialized using a uniform distribution, and a bias may not be used to reduce the number of parameters. L1 loss may be used, in lieu of L2 loss, as a cost function. The SR network suggested herein may be trained or learned using an Adam optimizer.

A learning rate may be set to be 0.0001 and be reduced by 10 for each 50 epoch. During the learning or training, a size of mini-batch may be set to be 2. For a learning or training test, a NVIDIA Titan X graphics processing unit (GPU) of 3.4 gigahertz (GHz) and an Intel Core i7-6700 central processing unit (CPU) may be used.

During calculation of a floating point in a learning or training stage, a weight parameter of the SR network may be quantized from the floating point to a fixed point according to Equation 2 in a test stage.

In addition, by quantizing activations of all convolution layers and using a compression method, only feature maps of 3×3 size DW convolution layers may be compressed. An optimal quantization bit for weights and activations may be experimentally discovered, and a quantized weight parameter may be used in the image processing device 10.

In an algorithm stage, a compressed intermediate feature map and a final SR image may be used as a golden model to be compared to a designed hardware simulation result.

FIG. 11 a illustrates an example of performance of the image processing device illustrated in FIG. 1 . FIG. 11 b illustrates another example of performance of the image processing device illustrated in FIG. 1 . FIG. 11 c illustrates still another example of performance of the image processing device illustrated in FIG. 1 .

Referring to FIGS. 11 a through 11 c , weight parameters and activations may be quantized for hardware implementation. The quantization of weight parameters and activations may greatly affect a quality of an output image, and thus it may need to discover a desirable quantization bit depth. That is, suitable values for three parameters described above—word length (WL), integer length (IL), and fraction length (FL)—may be required. Thus, experiments may be performed by varying such parameters WL, IL, and FL with respect to various datasets.

FIG. 11 a is a graph of PSNR with respect to WL and IL that quantize weight parameter values for a dataset, Set5. It is verified in FIG. 11 a that, when a bit depth is greater than or equal to 10, PSNR performance of an SR network may be similar to that in a case that there is no quantization of weights and activations. In addition, it is verified that the PSNR performance is reduced greatly at IL being 4 (IL = 4) or greater with respect to a 10-bit WL.

An FL bit depth may affect more greatly the PSNR performance than the IL bit depth, due to the use of a residual network as represented by Equation 5. For the quantization of weight parameters, the WL bit depth may be set to be 10 bit and the IL bit depth may be set to be 2 bit. This may also be used for the quantization of activations and the compression of intermediate feature maps.

FIG. 11 b is a graph of PSNR performance of an SR network with respect to WL and IL bit depths for the quantization of activations. Based on a result of an experiment as illustrated in FIG. 10 b , WL may be set to be 14 bit and IL may be set to be 2 bit for the quantization of activations.

FIG. 11 c illustrates a result of an experiment on a compression method applied to a quantized feature map to reduce the use of line memories. The experiment is performed to verify PSNR performance with respect to various block sizes and indices (quantization levels). As a value of a quantization level for the compression decreases, a compression ratio may increase, but the PSNR performance may decrease.

Based on a result illustrated in FIG. 10 c , a compromise between line memories requiring a 32-bit block size and a 5-bit index size (quantization level), and the result PSNR may be selected.

FIG. 12 a is an example of an original HR image. FIG. 12 b is an example of an image processed through a bicubic method. FIG. 12 c is an example of an image processed through an SRCNN method.

FIG. 12 d is an example of an image processed through an SRCNN-Ex method, an extension of SRCNN. FIG. 12 e is an example of an image processed through an FSRCNN method. FIG. 12 f is an example of an image processed through an FSRCNN-s method, a small model size version of FSRCNN.

FIG. 12 g is an example of an image processed through a VDSR method. FIG. 12 h is an example of an image processed by the image processing device illustrated in FIG. 1 using a quantized weight.

FIG. 12 i is an example of an image processed by the image processing device illustrated in FIG. 1 using a quantized weight and activation. FIG. 12 j is an example of an image processed by the image processing device illustrated in FIG. 1 using a quantized weight and activation, and compression of an intermediate feature map.

FIG. 13 a is another example of an original HR image. FIG. 13 b is another example of an image processed through a bicubic method. FIG. 13 c is another example of an image processed through an SRCNN method.

FIG. 13 d is another example of an image processed through an SRCNN-Ex method. FIG. 13 e is another example of an image processed through an FSRCNN method. FIG. 13 f is another example of an image processed through an FSRCNN-s method.

FIG. 13 g is another example of an image processed through a VDSR method. FIG. 13 h is another example of an image processed by the image processing device illustrated in FIG. 1 using a quantized weight.

FIG. 13 i is another example of an image processed by the image processing device illustrated in FIG. 1 using a quantized weight and activation. FIG. 13 j is another example of an image processed by the image processing device illustrated in FIG. 1 using a quantized weight and activation, and compression of an intermediate feature map.

Referring to FIGS. 12 a through 13 j , performance of the image processing device 10 is compared to a bicubic method and other CNN-based SR methods including SRCNN, SRCNN-Ex, FSRCNN, and FSRCNN-s based methods. A MATLAB (TM) source code which is publicly usable for SRCNN, SRCNN-Ex, FSRCNN, and FSRCNN-s may be used, and the image processing device 10 may be implemented using PyTorch.

For a fair comparison, boundaries of a reconstructed HR image and an original image may be excluded from PSNR and SSIM calculations. All the methods may be performed on a CPU platform.

An open code of the VDSR is executable only on a GPU platform, and thus a third-party code executed on the CPU platform may be used to measure PSNR and SSIM and a runtime.

A runtime of the image processing device 10 may be measured based on software implementation using PyTorch.

Table 4 illustrates mean PSNR and SSIM values of the SR method that are compared to four benchmark datasets.

TABLE 4 Part 1 of 2 Methods Bicubic SRCNN SRCNN-Ex FSRCNN FSRCNN-s Number of parameters - 8 K 57 K 12 K 4 K Weight bit - 32-bit 32-bit 32-bit 32-bit Activation bit - 32-bit 32-bit 32-bit 32-bit Dataset PSNR SSIM PSNR SSIM PSNR SSIM PSNR SSIM PSNR SSIM Set-5 33.66 0.9299 36.34 0.9521 36.66 0.9542 37.00 0.9557 36.57 0.9531 Set-14 30.24 0.8688 32.18 0.9039 32.42 0.9063 32.63 0.9086 32.28 0.9049 B100 29.56 0.8431 31.11 0.8835 31.36 0.8870 31.50 0.8909 31.23 0.8866 Urban100 26.88 0.8403 29.09 0.8897 29.50 0.8946 29.85 0.9010 29.23 0.8914

TABLE 4 Part 2 of 2 Methods VDSR Image processing apparatus (baseline) Image processing apparatus (W) Image processing apparatus Type-1 (W+A) Image processing apparatus Type-2 (W+A) Number of parameters 665 K 2.56 K Weight bit 32-bit 32-bit 10-bit 10-bit 10-bit Activation bit 32-bit 32-bit 32-bit 14-bit 14-bit Dataset PSNR SSIM PSNR SSIM PSNR SSIM PSNR SSIM PSNR SSIM Set-5 37.53 0.9587 36.66 0.9548 36.64 0.9544 36.64 0.9543 36.51 0.9520 Set-14 33.03 0.9124 32.52 0.9073 32.52 0.9071 32.47 0.9070 32.46 0.9055 B100 31.90 0.8960 31.32 0.8880 31.31 0.8876 31.31 0.8877 31.27 0.8864 Urban100 30.76 0.9140 29.34 0.8943 29.33 0.8942 29.32 0.8939 29.28 0.8916

Referring to Table 4 above, it is verified that the image processing device 10 has a greater level of performance compared to the FSRCNN-s, and occupies the number of filter parameters which is only 64% of that of the FSRCNN-s. In addition, it is verified that there is no performance degradation of the image processing device 10 even after quantization of weight parameter values and activations.

Although there is a performance degradation of approximately 0.1 dB in PSNR when applying feature map compression to a network of the image processing device 10, a required space of line memories may be reduced by a factor of approximately 2.58 times.

Table 5 illustrates a result of comparing the image processing device 10 and another CNN-based SR method in terms of an average computation time of PSNR and SSIM of test set-2 including a 4 K UHD test image.

TABLE 5 Methods Bicubic SRCNN SRCNN-Ex FSRCNN FSRCNN-s VDSR Image processing apparatus (baseline) Image processing apparatus (W) Image processing apparatus Type-1 (W+A) Image processing apparatus Type-2 (W+A) HW implementation FPGA, ASIC N/A N/A N/A N/A N/A N/A FPGA Average computation time (sec) - CPU GPU CPU GPU CPU GPU CPU GPU CPU GPU CPU GPU CPU GPU 0.0166 (60 fps) 277.6 1.052 288.0 1.256 324.4 0.583 146.4 0.518 124.3 2.851 2.53 0.050 2.62 0.057 Image PSNR SSIM PSNR SSIM PSNR SSIM PSNR SSIM PSNR SSIM PSNR SSIM PSNR SSIM PSNR SSIM PSNR SSIM PSNR SSIM Balloon 33.79 0.9409 35.37 0.9582 35.55 0.9594 35.74 0.9607 35.47 0.9591 35.99 0.9608 35.59 0.9601 95.589 0.9599 35.587 0.9599 35.550 0.9589 Children 33.56 0.9123 34.91 0.9272 34.97 0.9280 35.09 0.9291 34.92 0.9275 35.28 0.9296 35.00 0.9288 34.997 0.9283 34.995 0.9283 34.947 0.9265 Constance 31.98 0.9271 32.84 0.9436 32.94 0.9447 33.01 0.9460 32.88 0.9439 33.22 0.9470 32.93 0.9449 32.926 0.9446 32.924 0.9445 32.911 0.9440 Lake 30.1 0.8527 31.47 0.9004 31.68 0.9033 31.74 0.9047 31.58 0.9019 31.87 0.9057 31.57 0.9019 31.565 0.9016 31.561 0.9015 31.532 0.9006 Louvre 35.63 0.9476 38.2 0.9666 38.39 0.9677 38.93 0.9699 38.35 0.9673 39.36 0.9708 38.33 0.9680 38.320 0.9677 38.312 0.9676 38.234 0.9668 Medieval 29.68 0.9128 31.49 0.9424 31.76 0.9453 31.88 0.9464 31.60 0.9431 32.17 0.9482 31.61 0.9443 31.598 0.9437 31.596 0.9437 31.561 0.9427 High-rise building 29.48 0.9103 32.23 0.9434 32.75 0.9470 33.04 0.9488 32.67 0.9458 33.16 0.9496 32.57 0.9462 32.562 0.9458 32.550 0.9457 32.479 0.9439 Supercar 29.63 0.9453 32.22 0.9668 32.81 0.9699 33.00 0.9703 32.55 0.9679 33.20 0.9705 31.38 0.9681 32.370 0.9677 32.362 0.9676 32.308 0.9658 Average 31.74 0.9199 33.59 0.9436 33.86 0.9457 34.05 0.9470 33.75 0.9446 34.28 0.9477 33.75 0.9453 33.740 0.9449 33.736 0.9448 33.690 0.9437

It is verified that the image processing device 10 may reconstruct an HR image of a quality that equals to that obtained using the other SR methods. Regarding a runtime, a relatively longer runtime may be used because open codes of the SRCNN, the SRCNN-Ex, the FSRCNN, and the FSRCNN-s are implemented in MATLAB and may not be optimized on a CPU platform.

For a fair comparison, a network used by the image processing device 10 may also be implemented in TensorFlow, and other codes may be written in TensorFlow and a runtime may be measured in a GPU platform.

Referring to Table 5 above, runtimes measured by a GPU for various CNN-based SR methods including the image processing device 10 are verified. A runtime of the image processing device 10 run in the GPU is measured as approximately 50 ms, which may be faster about three times compared to FPGA implementation.

FIGS. 12 a through 12 j are images reconstructed using five CNN-based SR methods including a bicubic method and the image processing device 10, and cropped regions thereof. It is verified that, although the image processing device 10 uses the least number of parameters, a result HR image has a sharp edge and well-recognizable with less artifacts.

FIGS. 13 a through 13 j are cropped regions of an HR image reconstructed from a 4 K UHD resolution image of a child. It is verified that a visual quality obtained by the image processing device 10 and those obtained by other CNN-based SR methods are similar.

FIG. 14 illustrates an example of an implementation of SR hardware by an FPGA.

Referring to FIG. 14 , a prototype of the image processing device 10 implemented on an FPGA is verified. Table 6 illustrates details of implementation of methods by Lee and Yang, and super-interpolation (SI), and the implementation of the image processing device 10.

TABLE 6 Theses Lee Yang Kim Image processing apparatus Methods Sharp Filler Lagrange ANR Edge Orientation Learn Linear Mappings CNN Type-1 HW: No compression Type-2 HW: Compression FPGA device or CMOS technology 0.13 µm Altera EP4SGX530 90 nm Xilinx XCKU040 0.13 µm Xilinx XCKU040 FPGA resource or equivalence gate count* 5.1 K N/A 1,985 K Slice LUTs : 3.395 Slice Regs : 1,952 DSP Blocks: 108 159 K Slice LUTs : 110 K Slice Regs : 102 K DSP Blocks: 1920 Slice 151 K Slice Regs: 121 K DSP Blocks: 1920 Line buffer 4 (input) 5 (input), 24 (internal) 8 (output) 2 (input) 4 (output) 4 (input), 96 (internal). 4 (output) Memory size (Bytes) N/A 235 K 92 K 392 K Max frequency (MHz) 431 124.4 150 220 150 Throughput (Mpixels/s) 431 124.4 600 880 600 Supported scale 2X, 3X 2X 2X 2X PSNR (dB) Set5 N/A 33.83 34.78 36.64 Set14 N/A 29.77 31.63 32.47 Target resolution 4 K UHD (30 fps) FHD 160 fps) 4 K UHD (60 fps) 4 K UHD (60 fps)

^(∗)2 input NAND gates are counted as a single equivalence gate.

Lee et al. proposed hardware using a Lagrange interpolation method using a sharpening algorithm that may obtain a 4 K UHD video stream from an HD and FHD stream at 30 fps.

An HW architecture proposed by Yang may require an intermediate image of a target resolution to generate a high-frequency patch using a dictionary based on anchored neighborhood regression (ANR), and obtain an FHD at 60 fps.

A machine learning-based SI HW architecture may be based on linear mapping using an edge direction analysis that directly reconstructs an HR image through a high-frequency restoration without requiring an intermediate image.

The image processing device 10 may be implemented using SystemVerilog of the FPGA. An output clock speed of the image processing device 10 may be four times greater than an input clock speed thereof. This may be because an FHD (over 4 K UHD) operating frequency ratio is generally ¼.

The image processing device 10 may process four pixels per clock cycle and support a 4 K UHD video stream at 60 fps, and be implemented based on a restrictive condition applied to a stage of incorporating a 150 MHz target operating frequency and Vivado Design Suite (2015.4) and to a stage of placing and routing (P&R).

In addition. Xilinx Kintex UltraScale FPGA KCU105 evaluation board and TED’s HDMI 2.0 expansion card may be used to support an FHD input and a 4 K UHD output video interface to verify the implemented SR hardware.

Two types of SR HW may be provided to the image processing device 10. The two types may include Type-1 to which feature map compression is not applied, and Type-2 to which the feature map compression is applied.

In Type-1, a 110 K slice LUT and a 102 K slice register that occupy 45.38% of all slice LUTs and 21.08% of all slice registers in a XCKU040 FPGA device may be used.

In Type-2, a 151 K slice LUT and a 121 K slice register that occupy 62.6% of all the slice LUTs and 24.97% of all the slice registers may be used.

In addition, both Type-1 and Type-2 may maximally use a 1.920 DSP block in the XCKU040 FPGA device of the KCU105 evaluation board.

Type-2 may reduce on-chip memory usage, for example, block RAM in the FPGA, by approximately 50% of that of Type-1. Type-2 may further use approximately 38% of the slice LUTs and approximately 18% of the slice registers to implement two compressors 231 and six decompressors 235, compared to Type-1.

Although the image processing device 10 may require a greater number of line memories and gates compared to a non-CNN-based SR method, it may reconstruct a 4 K UHD HR image of a considerably higher quality in real time at a speed of 60 fps.

The units described herein may be implemented using hardware components and software components. For example, the hardware components may include microphones, amplifiers, band-pass filters, audio to digital convertors, non-transitory computer memory and processing devices. A processing device may be implemented using one or more general-purpose or special purpose computers, such as, for example, a processor, a controller and an arithmetic logic unit (ALU), a digital signal processor, a microcomputer, a field programmable gate array (FPGA), a programmable logic unit (PLU), a microprocessor or any other device capable of responding to and executing instructions in a defined manner. The processing device may run an operating system (OS) and one or more software applications that run on the OS. The processing device also may access, store, manipulate, process, and create data in response to execution of the software. For purpose of simplicity, the description of a processing device is used as singular; however, one skilled in the art will appreciated that a processing device may include multiple processing elements and multiple types of processing elements. For example, a processing device may include multiple processors or a processor and a controller. In addition, different processing configurations are possible, such a parallel processor.

The software may include a computer program, a piece of code, an instruction, or some combination thereof, to independently or collectively instruct or configure the processing device to operate as desired. Software and data may be embodied permanently or temporarily in any type of machine, component, physical or virtual equipment, computer storage medium or device, or in a propagated signal wave capable of providing instructions or data to or being interpreted by the processing device. The software also may be distributed over network coupled computer systems so that the software is stored and executed in a distributed fashion. The software and data may be stored by one or more non-transitory computer readable recording mediums. The non-transitory computer readable recording medium may include any data storage device that can store data which can be thereafter read by a computer system or processing device.

While this disclosure includes specific examples, it will be apparent to one of ordinary skill in the art that various changes in form and details may be made in these examples without departing from the spirit and scope of the claims and their equivalents. The examples described herein are to be considered in a descriptive sense only, and not for purposes of limitation. Descriptions of features or aspects in each example are to be considered as being applicable to similar features or aspects in other examples. Suitable results may be achieved if the described techniques are performed in a different order, and/or if components in a described system, architecture, device, or circuit are combined in a different manner and/or replaced or supplemented by other components or their equivalents.

Therefore, the scope of the disclosure is defined not by the detailed description, but by the claims and their equivalents, and all variations within the scope of the claims and their equivalents are to be construed as being included in the disclosure. 

What is claimed is:
 1. An image processing device comprising: a receiver configured to receive an image; at least one first line buffer configured to output the image as a line-wise image line; a first convolution operator configured to generate a feature map by performing a convolution operation based on an output of the first line buffer; and a feature map processor configured to store an output of the first convolution operator in at least one line-wise unit, and process a feature map stored in at least one line-wise unit to be output in a two-dimensional (2D) form, wherein at least one convolution operation is performed in a form of a pipeline.
 2. The image processing device of claim 1, wherein the first convolution operator is implemented in a residual block that learns a residual signal and outputs the learned residual signal.
 3. The image processing device of claim 1, wherein the first convolution operator comprises: at least one one-dimensional (1D) convolution operator configured to perform an 1D convolution operation.
 4. The image processing device of claim 1, wherein the first convolution operator comprises: a depth-wise convolution operator; and a point-wise convolution operator connected directly to the depth-wise convolution operator.
 5. The image processing device of claim 1, wherein the feature map processor comprises: a compressor configured to compress the output of the first convolution operator into at least one line-wise unit.
 6. The image processing device of claim 5, wherein the feature map processor further comprises: at least one second line buffer configured to store the output of the first convolution operator that is compressed line-wise.
 7. The image processing device of claim 6, wherein the feature map processor further comprises: a decompressor configured to reconstruct an output of the second line buffer to a 2D feature map.
 8. The image processing device of claim 1, further comprising: a second convolution operator configured to perform a convolution operation based on an output of the feature map processor.
 9. The image processing device of claim 8, wherein the second convolution operator comprises: at least one 2D convolution operator configured to perform a 2D convolution operation.
 10. The image processing device of claim 8, wherein the second convolution operator comprises: a depth-wise convolution operator; and a point-wise convolution operator connected directly to the depth-wise convolution operator.
 11. The image processing device of claim 1, further comprising: a quantizer configured to quantize a result of at least one convolution operation.
 12. The image processing device of claim 1, further comprising: a weight buffer configured to store a parameter used for a convolution operation.
 13. An image processing method comprising: receiving an image; outputting the image as a line-wise image line, through at least one first line buffer; generating a feature map by performing a first convolution operation based on an output of the first line buffer, through a first convolution operator; and storing an output of the first convolution operator in at least one line-wise unit and processing a feature map stored in at least one line-wise unit to be output in a two-dimensional (2D) form, through a feature map processor, wherein at least one convolution operation is performed in a form of a pipeline.
 14. The image processing method of claim 13, wherein the first convolution operation is performed in a residual block that learns a residual signal and outputs the learned residual signal.
 15. The image processing method of claim 13, wherein the generating comprises: performing at least one one-dimensional (1D) convolution operation.
 16. The image processing method of claim 13, wherein the generating comprises: performing a depth-wise convolution operation; and performing a point-wise convolution operation directly on a result of the depth-wise convolution operation.
 17. The image processing method of claim 13, wherein the processing comprises: compressing the output of the first convolution operator into at least one line-wise unit.
 18. The image processing method of claim 17, wherein the processing further comprises: storing, through at least one second line buffer, the output of the first convolution operator that is compressed line-wise.
 19. The image processing method of claim 18, wherein the processing further comprises: reconstructing an output of the second line buffer to a 2D feature map.
 20. The image processing method of claim 13, further comprising: performing a second convolution operation based on an output of the feature map processor. 